For all the complexity involved in harmonising more than 50 different benefit payments into one universal credit, Iain Duncan Smith's welfare reform bill draws us into a moral debate that, if not as old as the hills, is certainly as old as the mills: What is the appropriate relationship between individual and state? Where is the boundary between responsibility for myself and responsibility for others?

Although our answer to such questions eddies and drifts on the surface of circumstance, both personal and national, there is a discernible pattern to the national response. Two hundred years ago, clergyman Thomas Malthus wrote in his Essay on the Principle of Population:

"If any man choose to marry, without a prospect of being able to support a family, he should have the most perfect liberty so to do … [but] he should be taught to know, that the laws of nature, which are the laws of God, had doomed him and his family to suffer for disobeying their repeated admonitions; [and] that he had no claim of right on society for the smallest portion of food."

Few would be so harsh or so openly moral (let alone so confidently theological) today, but we deceive ourselves if we think the issue before us is qualitatively different.

The very question over what a workless family has a right to receive from society – the welfare bill proposes "a cap on the total amount of benefit that working-age people can receive so that workless households will no longer receive more in benefit than working households receive in average wages" – seems almost chosen with Malthus's example in mind.

Malthus and his essay were hugely influential, not only on Darwin (alerting him to the mechanics of evolution) but also on those who designed the 1834 Poor Law Amendment Act, which abolished outdoor relief and set up the infamous workhouses.

Basing his thinking on the same hubristic, rational theology that saw William Paley confidently read God from nature, Malthus saw the world as essentially just. Those who fell had no one but themselves to blame. Stung by accusations of callousness, he repeatedly insisted that personal charity and benevolence were mandatory. But he never shifted from the conviction that political economy had revealed creation's moral laws, just as Newton had revealed its physical ones, and that these were intended not so much to improve our lot as to improve our character.

It was such ideas, albeit much moderated, that informed Margaret Thatcher's thinking, with its insistence that politics and economics were always situated within, and should defer to, wider moral and spiritual concerns. "The role of the state in Christian society is to encourage virtue, not to usurp it," she told an audience at St Lawrence Jewry in 1978. In between Malthus and Thatcher stretches the arc of British social welfarism. As the 19th-century progressed, more and more Britons recognised that society wasn't entirely fair, that the laws of economics were not necessarily the laws of nature (let alone of God) and that accident and circumstance could be as (or more) culpable for human destitution as personal moral failure. In the 20th century, the nation did something about it, and virtually everyone rejoiced. "Christians should welcome the welfare state as the embodiment of the principle, 'Bear ye one another's burdens'," wrote Cyril Garbett, Archbishop of York, in 1952.

For all she would have disagreed with Garbett, Thatcher was no Malthusian. Nevertheless, her rhetoric, reforms and electoral success traced the new direction in attitudes to welfare, nudging the boundary between personal and state responsibility back whence it had come.

There is widespread recognition today that welfare needs reform, not only because it is complex but also because, as Duncan Smith said in a Theos Charities Parliament lecture, it has long, if unintentionally, offered too much stick and not enough carrot. This strongly suggests that the border line will continue to shift: away from the corporate, state-based responsibility that reached its zenith in the 1940s and towards individualised, self-responsibility.

Perhaps the most discomfiting thing is that the change is not against the tide of public opinion. If anything, it is behind it. Over the last quarter century, the proportion of people who believe unemployment benefit is too high has risen from 35% in 1983 to 61% in 2008. The proportion of people who agree that "if welfare benefits weren't so generous, people would learn to stand on their own two feet" has increased from 33% in 1987 to 53% in 2008. And the proportion of people who agree that "the government should spend more money on welfare benefits for the poor, even if it leads to higher taxes" has fallen from 55% in 1983 to 35% in 2008.

This renewed public emphasis on personal responsibility may include personal responsibility for others alongside responsibility for myself, the kind of civic engagement that is central to the "big society" and "red" Toryism. That would be good.

Alternatively, it may just signify a shift towards narrow self-interest, in which we expect the poor and unemployed to stand on their own feet and decry any sense of responsibility towards them, whether personal or statutory. And if this is the case, we should all be worried.